Cancer is a genetic disease that results from multiple genomic changes. These changes ultimately lead to the malfunction of cell cycle machinery and finally to autonomous cell proliferation. Neoplastic transformation involves four types of genes: oncogenes, tumor-suppressor genes, mutator genes, and apoptotic genes. Different types of cancer can involve alteration of any one or any combination of these genes.
In order for genomic alterations to lead to effects on cell proliferation, they must in some way affect the normal processes that control the cell cycle. The cell then becomes an autonomous unit capable of replicating without reliance on the external environment that normally signals a cell when to divide, to arrest, to differentiate, or to apoptose. Therefore, neoplastic development is ultimately a disturbance in either the positive regulators of the cell cycle (proto-oncogenes), the negative regulators (tumor suppressor genes), and/or apoptotic regulators of cell growth and proliferation. To develop more effective strategies for identification and treatment of various forms of cancer the mechanisms that operate within the cell cycle and how they might interlink, become disturbed, and be regulated must be considered.
Major advances in molecular biology, biochemistry and tumor biology have changed the way research scientists and clinicians conceptualize the management and treatment of cancer. Proteins that mediate cell cycle control are now being elucidated (e.g., p53 nuclear protein), while DNA technology provides ready access to the genes that control the events. Direct targeting of these proteins for either delivery or modification by conventional pharmacologic agents is difficult because of the size, inaccessibility, and complexity of the proteins themselves. Manipulation of the genes that control expression of these proteins, i.e., gene therapy, however, overcomes these barriers by selectively introducing recombinant DNA into tissues thereby leading to alterations in the expression of the biologically active protein in tissues. This in turn leads to alterations in cell function.
Gene therapy is now accepted as a therapeutic tool for treating diseases such as cancer. See, for example, Eck and Wilson Goodman and Gilman""s The Pharmacological Basis of Therapeutics 1996, Chapter 5. Gene therapy for treatment of cancer has been the focus of multiple clinical trials approved by the National Institutes of Health Recombinant DNA Advisory Committee, many of which have demonstrated successful clinical application (Hanania et al. Am. Jour. Med. 1995 99:537-552; Johnson et al. J. Am. Acad. Derm. 1995 32(5) :689-707; Barnes et al. Obstetrics and Gynecology 1997 89:145-155; Davis et al. Current Opinion in Oncology 1996 8:499-508; Roth and Cristiano J. Natl. Canc. Inst. 1997 89(1):21-39). An important factor for success when employing alterations of gene expression as a therapeutic strategy is to begin by establishing a sound scientific basis for manipulation of that gene and its gene product.
The focus of gene therapy strategies for cancer has been not only on altering growth of a malignancy but also on diagnosis and prognosis of patients"" disease. For diagnostic and prognostic methods, markers for cell cycle regulatory proteins are being developed. A goal of these methods is to decrease the morbidity and mortality of various forms of cancer by promoting earlier recognition and intervention. Early detection of tumors allows for use of more effective treatments such as immunotherapy which requires a low tumor burden ( less than 109 cells). Further, more efficient classification of tumors based on molecular alterations in oncogenes, tumor suppressor genes, apoptotic genes, and/or mutator genes allows for less subjective interpretation by pathologists. This in turn leads to better choices of therapy.
Many forms of malignancy have been linked to mutations in the tumor suppressor gene known as retinoblastoma, or Rb. The retinoblastoma gene which encodes the nuclear protein p53 is the most frequently altered gene in cancer, where defects in the function of this suppressor gene lead to unregulated cell proliferation. Studies have led to the identification of an additional member of the Rb family, p107 which has also been labeled as a tumor suppressor gene. Cellular proliferation has been shown to be triggered by growth factor receptor activation as well as being controlled by cell cycle regulators such as Rb and p107 (DeCaprio et al. 1989 Cell 58:1085-1095; Zhu et al. 1993 Gene Develop. 7:1111-1125).
A third gene, Rb2/p130, has also been identified which is structurally and functionally related to both the Rb gene itself and p107 (Baldi et al. Proc. Natl. Acad. Sci. USA 1996 93:4629-4632). All three share several regions of identity that constitute a functional domain known as the xe2x80x9cpocket regionxe2x80x9d. This xe2x80x9cpocket regionxe2x80x9d is involved in binding to the transforming proteins from DNA tumor viruses, such as the ElA protein from adenovirus, as well as binding to cellular transcription factors such as E2F. Therefore, the xe2x80x9cpocketxe2x80x9d plays a critical role in protein-protein interactions.
pRb2/p130 has been cloned and identified based upon binding to the E1A transforming domain (Mayol et al. 1993, Oncogene 8:2561-2566). Further, the genomic structure of the human retinoblastoma-related Rb2/p130 gene which provides a molecular basis for understanding the transcriptional control of the gene itself and for delineating potential Rb2/p130 mutations in human tumors was recently disclosed (Baldi et al. 1996 Proc. Natl. Acad. Sci. 93:4629-4632). The Rb2/p130 gene has been mapped to human chromosome 16q12.2 (Yeung et al. 1993 Oncogene 8:3465-3468); deletions of this chromosome have been found in several human neoplasias including breast, hepatic, ovarian, and prostatic cancer. Accordingly, pRb2/p130 is believed to be a tumor suppressor gene in human carcinoma.
Rb2/p130 has been shown to have a role as regulator in cell cycle function. For example, Baldi et al. have shown that phosphorylation of the Rb2/p130 gene product is regulated in a cell cycle dependent manner (Baldi et al. 1995 J. Cell. Biochem. 59:402-408), in the same way that the phosphorylation of Rb is cell cycle dependent (DeCaprio et al. 1989 Cell 58:1085-1095). Further, the growth suppressive properties of the gene product of Rb2/p130 have been shown to be specific for the G1 phase in similar fashion to pRb and p107 (Claudio et al. 1996 Cancer Res. 56:2003-2008). The gene product of Rb2/p130 has been shown to arrest growth in human tumor cell lines in a manner similar to the other members of the Rb family (i.e., pRb and p107). However, this protein also inhibits proliferation in a glioblastoma cell line that is resistant to the growth suppressant effects of both pRb and p107 (Claudio et al. 1994 Cancer Res. 54:5556-5560). Accordingly, pRb2/p130 has similar yet distinctive growth suppressive properties from pRb and p107 (Claudio et al. 1994 Cancer Res. 54:5556-5560).
It has now been found that pRb2/p130 affects tumor cell growth. The tumorigenicity mediated by a specific oncogene, c-erbB-2 (HER2 neu) is affected. Further, it has been found that pRb2/p130 affects tumor cell growth in cells not linked to expression of c-erb-2, as well as in tumor cells that are deficient in pRb2/p130 and cells that contain pRb2/p130. Compositions which comprise pRb2/p130 have been identified that can be incorporated into a vector and transfected into tumor cells to inhibit growth of the cells.
An object of the present invention is to provide a composition comprising a vector expressing pRb2/p130 and a pharmaceutically acceptable carrier.
Another object of the present invention is to provide a method of inhibiting growth of tumor cells which comprises transfecting tumor cells with a plasmid comprising pRb2/p130.
Another object of the present invention is to provide a method for diagnostic screening of tumor cells to identify those tumors best treated by administration of a plasmid comprising pRb2/p130.
The present invention relates to compositions and methods for inhibiting tumorigenicity. The compositions and methods relate to reversing the tumorigenicity of pRb2/p130 negative cancer cells and cells induced by proto-oncogenes such as c-erbB-2, as well as cells that express pRb2/p130. In the present invention, a composition comprising a vector expressing pRb2/p130 is transfected into tumor cells so that growth of the tumor cells is inhibited.
Carcinomas of the breast, ovary, lung, and stomach each have been linked to the presence and expression of a specific human proto-oncogene known as c-erbB-2, also known as HER2/neu (Slamon et al. 1989 Science 244:707-712; Park et al. 1989 Cancer Res. 49:6605-6609; Rilke et al. 1991 Int. J. Cancer 49:44-49; Guy et al. 1992 Proc. Natl. Acad. Sci. 89:10578-10582; Kern et al. 1994 J. Clin. Invest. 93:516-520; Pupa et al. 1996 J. Clin. Oncol. 14:85-94). The c-erbB-2 (HER2 neu) oncogene encodes a 185,000 dalton epidermal growth factor receptor-related transmembrane protein (p185HER2) that has intrinsic tyrosine kinase activity (Bargmann et al. 1986 Nature 319:226-230; Coussens et al. 1985 Science 230:1132-1139). Overexpression of this protein correlates with aggressive disease and a poor prognosis (Rilke et al. 1991 Int. J. Cancer 49:44-49; Kern et al. 1994 J. Clin. Invest. 93:516-520; Pupa et al. 1996 J. Clin. Oncol. 14:85-94). Specifically, amplification of this gene has been associated with breast cancer (Slamon et al. 1989. Science 244:707-712), ovarian cancer (Slamon et al. 1989. Science 244:707-712), and primary stomach carcinoma (Park et al. 1989. Cancer Res. 49:6605-6609). Expression of c-erbB-2 has also been suggested to be related to the metastatic qualities of tumor growth based upon studies in a mammary tumor model in mice (Guy et al. Proc Natl. Acad. Sci. USA 1992 89:10578-10582) and lung carcinomas from humans (Kern et al. J. Clin. Invest. 1994 93:516-520). Thus, c-erbB-2 expression plays a role in the development and progression of several forms of carcinoma in both animals and humans.
It has now been found that introduction of the pRb2/p130 cDNA into human cancer cells (those specifically linked to c-erbB-2 expression) results in reduction of growth rate and colony formation in vitro and suppresses tumor growth in vivo in athymic mice, despite the expression of p185HER2 receptor capable of autophosphorylation. In addition, studies in tumor cell lines not specifically linked to c-erb-2 expression have shown that pRb2/p130 expression inhibits tumor cell growth. Studies were performed in three different types of tumor cells, ovarian tumor cells, lung tumor cells, and brain tumor cells, as well as in patients with endometrial cancer.
Plasmid containing the coding sequence for pRb2/p130 under the transcriptional control of CMV promoter and an empty vector (mock transfection) were successfully transfected into SKOV3 cells. Northern blot analysis demonstrated an increase in pRb2/p130 mRNA levels in the transfected clones as compared with mock and wild-type SKOV3 cells. The increase was confirmed at the protein level by Western Blot analysis. The kinase activity associated with the p185HER2 molecule indicated that the receptor was functional in all cell populations tested. The phosphorylation status of the protein, estimated by western blot with anti-phosphotyrosine antibody of anti-p185HER2 immunoprecipitates, indicated that there were similar levels in clones and controls.
The effect of pRb2/p130 ectopic expression on SKOV3 cell growth was evaluated initially by colony formation assay. Results showed that pRb2/p130-transfected cells gave rise to a lower number of colonies than the mock-transfected cells. Further, the rate of growth was significantly decreased in comparison to that of the mock control cells (p less than 0.00l), with decreases ranging from 30% to 54%. The soft agar colony formation assay indicated a significant cloning formation inhibition for each pRb2/p130 transfectant tested compared with that of mock-transfected cells (p less than 0.001). The number of foci originating from the clones decreased from 84% to 94% in comparison to mock-transfected cells.
In vivo injection of the transfected cells led to inhibition of tumorigenicity of SKOV3 cells. The tumor volume evaluated at 56 days from injection indicated a significant reduction in proliferation of the tumors originating from pRb2/p130 transfected cells when compared with tumors originating from mock-transfected cells. This is the first demonstration that Rb2/p130 can act in vivo as a tumor suppressor gene in cell lines known to be associated with expression of c-erbB-2.
Considered together, these data are clear demonstration of the ability of pRb2/p130 to inhibit cell growth by mechanisms involving specific effects on c-erbB-2 gene products. pRb2/p130 is believed to act as an oncosuppressor molecule in cells over expressing c-erbB-2. Thus, expression of pRb2/p130 provides a method for suppressing tumorigenesis induced by c-erbB-2 in a variety of human cancers including but not limited to human glioblastoma, melanoma, breast, lung, endometrial, and stomach carcinomas.
The effects of pRb2/p130 on growth of tumor cells not deficient in pRb2/p130 or not known to be induced by c-erb-2, i.e., a tumor cell line containing a mutated Rb2/p130 gene, were also examined. In experiments with aggressive human lung carcinoma cell lines (H23 and A549), a murine leukemia-based (MLV-based) retroviral vector system was used for efficient delivery of the wild type Rb2/p130 gene. The H23 cell line has a mutant Rb2/p130 locus while A549 has a wild-type locus. Cells were transduced with empty virus (mock) or retroviruses carrying the puromycin resistance gene alone (MSCVPac) or in combination with pRb2/p130 (MSCVPac pRb2/p130) and selected for puromycin for 10 days. The cells transduced with the retroviruses transferring Rb2/p130 were severely growth suppressed in both the H23 and A549 cell lines. The cells were also transduced with a serial dilution of the supernatant retroviruses. The growth suppressive effects of retroviral delivery of the Rb2/p130 gene were found to be dose dependent. These data demonstrate that the growth inhibitory effects of upregulated expression of pRb2/p130 not only include Rb2/p130-deficient tumor cells but also those with wild-type Rb2/p130. In addition, these data show that the mutant pRb2/p130 protein produced in the H23 cell line does not behave in a dominant negative manner as it is unable to inhibit the growth suppressive activity of the native protein.
To confirm that the growth suppressive properties of pRb2/p130 were due to specific up-regulation of pRb2/p130 protein levels, H23 cells were transduced with empty retroviral vector or retroviruses carrying the Rb2/p130 gene. Transduction with viruses carrying the Rb2/p130 cDNA led to a significant increase in pRb2/p130 protein levels. Similar results were found with the A549 cell line that contained the wild-type pRb2/p130 protein.
The neoplastic properties of the H23 cell line following transduction with the retrovirus MSCVPac, either alone or in combination with pRb2/p130, were assessed by the ability to form colonies in soft agar. H23 cells were transduced and selected for 15 days in puromycin than seeded in duplicates into 0.3% agarose with puromycin. After 3 weeks, colonies larger than 50 cells were scored. The colony forming potential of H23 cells was significantly decreased in cells containing the Rb2/p130 transgene as compared to pooled clones transduced with MSCVPac alone. The size of the colonies was decreased by 5- to 6-fold; Rb2/p130 colonies were 82% smaller than Pac colonies. The decrease in colony number with transduction of the tumor suppressor gene was 10-fold, or 90.4%.
The effects of retroviral delivery of pRb2/p130 on tumor formation both ex vivo and in vivo were examined in nude mice. H23 tumor cells were transduced in culture with retroviruses carrying only the Pac resistance gene, or Pac and the Rb2/p130 cDNA, and selected for 15 days. Equal numbers of MSCVPac and MSCVPac-Rb2/p130 transduced cells were injected into the flank of nude mice. Transduction with MSCVPac-Rb2/p130 greatly suppressed the ability of the cells to form tumors as compared to animals injected with MSCVPac-transduced cells. There was approximately a 20-fold (99.5%) reduction in tumor-forming potential (based on average tumor weight of excised tumors) Similar results were seen when the study was repeated. For in vivo transduction of the gene, nude mice were injected with 2.5xc3x97106 H23 cells and tumors were allowed to grow for 15 days. After 15 days the tumors were transduced with retroviruses carrying either MSCVPac alone or MSCVPac-Rb2/p130, or the bacterial beta-galactosidase (LacZ) gene. Transduction with LacZ did not affect tumor growth rate or tumor-forming potential as compared to animals injected with MSCVPac alone. However, treatment with MSCVPac-Rb2/p130 suppressed tumor forming potential of the H23 cells as compared to that of either LacZ or MSCVPac treatment groups. The reduction in tumor growth with MSCVPac-Rb2/p130 transduction was more than 12-fold (92%). In fact, the tumors transduced with Rb2/p130 retrovirus diminished in size after a single injection, and 4 of 6 tumors completely regressed. The volume of tumors transduced with Rb2/p130 retrovirus was 4.6-fold less (79% decrease) as compared to the other treatments. Examination of the tumors formed in animals treated with the retroviral vectors were typical of lung adenocarcinoma and more than 75% of the tumor cells were highly positive for pRb2/p130 expression by immunohistochemistry of tumors transduced with the MSCVPac-Rb2/p130 vector. The suppression of tumor formation and progression in both ex vivo and in vivo transduction studies was dependent on induction of pRb2/p130 expression in the tumor cells as demonstrated through Western blot analysis of tumor cell lysates. These in vivo and ex vivo data confirm the results of the in vitro studies and show that expression of Rb2/p130 gene in tumor cells leads to an inhibition of tumor cell growth and suppression of tumor formation in animals, even in tumor cells that are not deficient in pRb2/p130.
Experiments were also performed to examine the role of the pRb2/p130 gene in brain tumor growth. A modified tetracycline-regulated method was used to create an autoregulatory inducible Rb2/p130 gene expression system in the HJC-15c cell line, one of five clonal cell lines originating from a human polyomavirus-induced hamster brain tumor (Shockett, P. et al. 1995. Proc. Natl. Acad. Sci. USA 92:6522-6526). This cell line was chosen because it exhibited the lowest expression of Rb2/p130 mRNA and the shortest doubling time of the five clones (HJC-15a, b, c, d, and e). The parental cell line was used to create the control cell line HJCxcex945 which contains the tetracycline transactivator (tTA) under the control of the Tetp promoter. HJCxcex945 cells were used to form the HJC12 cell line which contains, in addition to tTA, the full length cDNA of the human Rb2/p130 gene downstream of the Tetp promoter. In this system, expression of Rb2/p130 is repressed in the presence of the antibiotic tetracycline and induced in its absence. Results showed that expression of Rb2/p130 is enhanced at the mRNA level and 160 fold at the protein level upon withdrawal of tetracycline from the medium. Cytochemistry determined that expression of Rb2/p130 remained in the nucleus upon induction with tetracycline.
Equal numbers of HJC12 cells were then plated in medium containing or lacking tetracycline and were grown for 15 days. Induction of pRb2/p130 resulted in a dramatic growth arrest of the cells as evidenced by colony assay. HJC12 and HJCxcex945 cells were each plated in the presence or absence of tetracycline for 24 hours then harvested. Their cell cycle profile was examined by flow cytometry which showed that upon induction of pRb2/p130 expression, nearly 90% of cells were found in the G0/G1 stage corresponding to a mean increase of 41.6% in the G0/G1 population. These results are in agreement with data from transient transfection experiments in other human cell lines wherein it was demonstrated that the growth suppressive action of pRb2/p130 is specific to the G0/G1 phase of the cell cycle (Claudio, P. P. et al. 1996. Cancer Res. 56:2003-2008). This effect was specific for pRb2/p130 expression since withdrawal of tetracycline from the HJCxcex945 parental cell line did not affect their cell cycle distribution.
Experiments were then performed to determine if JCV TAg, like its SV40 counterpart, interacted with pRb2/p130. It was first confirmed that the cells line HJCxcex945 and HJC12 were transformed by JCV and not cross-contaminated with SV40 TAg. Immunoprecipitation of HJC cell lines xcex945 and 12 grown in the presence and absence of tetracycline with anti-pRb2/p130 immune serum followed by Western blotting analysis with PAB 416 detected a physical interaction between pRb2/p130 and JCV TAg.
Combined immunoprecipitation/Western blot analysis using PAB 416/anti-pRb2/p130, respectively, also detected this complex. This association increased upon induction of pRb2/p130 expression in HJC12 cells grown without tetracycline.
Since the function of pRb2/p130 is believed to be modulated through phosphorylation, the phosphorylation status in the JCV TAg complex was examined. The proteins in the HJC12 cells grown in the absence and presence of tetracycline were labeled with 32P orthophosphate; immunoprecipitated with normal rabbit serum, pRb2/p130 immune serum, or PAB 416; and then run on a 8% SDS-polyacrylamide gel. Anti-pRb2/p130 precipitated pRb2/p130 with only the hyperphosphorylated form being visualized and 92 kDa phosphorylated form of JCV TAg. PAB 416 immunoprecipitated both the 88 and 92 kDa phosphoforms of JCV TAg but failed to precipitate pRb2/p130. Thus, the 92 kDa form of JCV TAg associates with hypophosphorylated pRb2/p130, but fails to bind hyperphosphorylated pRb2/p130. This association was detected in the HJC12 cells in both the uninduced and induced states, as well as in the HJCxcex945 cells, demonstrating that this association is specific and not merely due to inordinately high levels of pRb2/p130 protein and that neither tetracycline nor the tetracycline transactivator (tTA) affect complex formation.
The ability of induction of pRb2/p130 to overcome JCV TAg-mediated transformation and tumor formation in vivo was then examined. Nude mice were injected with either HJCxcex945 or HJC12 cells (5xc3x97106 cells) in each flank and then administered a solution of 1 mg/kg of active tetracycline hydrochloride and 1% sucrose with tetracycline or 1% sucrose without tetracycline in their drinking water. The study was performed twice. The induction of pRb2/p130 expression in the mice injected with HJC12 without tetracycline greatly retarded the rate of tumor growth as compared to mice injected with HJC12 with tetracycline. No statistically significant difference was found between the HJCxcex945 groups (with and without tetracycline) There was at least some growth of the cells in each of the mice demonstrating that the cells injected were viable and that the growth inhibitory effects were not due to nonviability of the cells prior to or during the injection process. At the end of two replicate experiments, the combined weights of the excised tumors from each of the HJC12 groups were compared. No significant difference was found in tumor weights of HJCxcex945 groups between the two experiments. HJCxcex945 tumors grew faster than HJC12 tumors and those mice had to be sacrificed before the HJC12 mice. All HJC12 mice were sacrificed at the same time in each study. In the first study, induction of pRb2/p130 expression retarded tumor growth by 2.9-fold or 66%. In the second study induction of pRb2/p130 expression inhibited tumor growth by 3.4-fold or 71%. The average effect of pRb2/p130 induction in the two replicate experiments was 3.2-fold or 69%, a statistically significant effect on tumor growth as compared to the control groups. A third experiment was performed where tetracycline was administered through subcutaneous implantation of a 42 mg, 60 day, time-release pellet designed to deliver 0.7 mg tetracycline or placebo per day. Results were similar, with significant differences in tumor growth evident upon induction of pRb2/p130 expression in HJC12 cells (a 4.3-fold inhibition or 77% inhibition of tumor growth) . Western blot analysis confirmed that the tumor growth was dependent upon an increase in pRb2/p130 expression in vivo by examining protein extracts of the tumor specimens. The expression of pRb2/p130 and JCV TAg both remained nuclear upon induction of pRb2/p130 as confirmed by cytochemistry. Induction of pRb2/p130 expression and co-localization within the nuclei of the protein with JCV TAg in vivo was also demonstrated by immunohistochemistry of parallel tumor sections and cytochemistry.
These data show that the inducible system of pRb2/p130 expression functions both in vitro and in vivo to inhibit tumor cell growth in a human brain tumor cell line. These results are consistent with the studies in lung tumor cell lines and ovarian tumor cell lines.
Experiments were performed to examine the application of methods of the present invention in identifying tumor cell types. In the first studies, the relationship between pRb2/p130 expression and endometrial cancer was examined. In 100 patients that had not received treatment prior to surgery, it was found that low levels of expression of this gene were associated with poor clinical prognosis. Further, tumors of these patients behaved more aggressively. The risk of dying of disease was five times greater in patients whose tumors were pRb2/p130 negative.
Screening of a panel of human tumor cells and primary Burkitt""s lymphoma and nasopharyngeal tumors resulted in identification of a common site of mutation in the Rb2/p130 gene which results in nuclear exclusion of this protein. The expression of Rb2/p130 by immunocytochemistry and western blot analysis was determined in four osteosarcoma and in four lymphoid tumor cell lines using the polyclonal anti-Rb2/p130 antibody, ADLI. Cytoplasmic localization of Rb2/p130 was found in all lymphoid tumor cell lines, while all osteosarcoma cell lines showed an exclusive nuclear localization. Western blot analysis on nuclear and cytoplasmic fractions of all samples showed no substantial alteration in the molecular weight of the protein and confirmed the exclusive cytoplasmic or nuclear localization in the two types of neoplastic cell lines.
Accordingly, dependence of intracellular distribution of the protein upon mutations which differently affect the nuclear localization signal (NLS) motifs was verified. In these experiments, the structure of exons 19 through 22 of the Rb2/p130 gene which encode for the B domain and the C terminus of the protein were studied. Genomic DNA sequences from coding exons 19 through 22 were amplified and screened for mutation by SSCP analysis.
Both the direct PCR products and the DNA obtained from cutting the bands with altered migration patterns from SSCP gels were sequenced in order to identify the actual mutations. Evaluations of the sequencing chromatograms and the SSCP gels were used to determine whether or not mutations were heterozygous or homozygous. CCRF-CEM and Jurkat cell lines showed insertions in exon 22, where the NLS is located, causing a frameshift that resulted in the loss of the NLS. The Daudi cell line showed an insertion in exon 22 that occurred downstream from the putative NLS however, an insertion in exon 21 of Daudi cells effectively causes the loss of the NLS present in exon 22 by the resulting shift in the coding frame. Similarly, no mutations or insertions were observed in exon 22 of the Molt-4 cell line; however, an insertion in exon 21 gave a frameshift causing the loss of the bipartite NLS located in exon 22. In contrast, all osteosarcoma cell lines exhibited only point mutations in exons 19, 20 and 21 which did not alter the NLS.
The functional consequences of disruptions in the NLS of pRb2/p130 were then determined. Experiments were first performed to determine whether or not this region can serve in and of itself as a NLS. The NLS of pRb2/p130 from amino acids 1082 to 1102 was fused to the amino-terminus of EGFP in the pEGFP-N1 [pEGFP-N1-NLS] expression vector which expresses a human codon-optimized, red-shifted green fluorescent protein that can be fused to heterologous proteins serving as a fluorescent tag (Clontech Laboratories, Inc., Palo Alto, Calif.). EGFP is a low molecular weight protein that lacks any localization signal and is equally distributed in the nuclear and cytoplasmic compartments. Point mutations were also constructed in the first region of the bipartite NLS resulting in a change in amino acids K-1082 to N and R-1083 to Q [pEGFP-N1-NLS-NQ1], in the second site K-1100 to N and R-1102 to Q [pEGFP-N1-NLA-NQ2], as well as the combination of mutations in both bipartite sites [pEGFP-N1-NLS-NQ1and2]. The plasmids were transfected into Saos-2 cells, and the locations of the ectopically expressed proteins were determined by fluorescence microscopy. Saos-2 cells were chosen because Rb2/p130 is found to be exclusively nuclear in the cells confirming that they do not harbor any mutations in other proteins which may affect nuclear shuttling directed by this regions. The cells were counterstained with propidium iodide. In pEGFP-N1 transfected cells, EGFP was expressed ubiquitously in the cell. Fusion of the wild-type bipartite NLS to EGFP localized expression exclusively in the nucleus. EGFP fused to point mutations in either the upstream region or downstream region of the bipartite NLS resulted in mostly nuclear expression. Mutation in both bipartite sites resulted in an expression pattern without any nuclear targeting wherein the fusion protein equilibrated between the nuclear and cytoplasmic compartments in similar fashion to cells transfected with pEGFP-N1. Accordingly, this region of pRb2/p130 serves as a bipartite NLS where both the upstream and the downstream signals can independently dictate nuclear expression. Only combined mutations in both the bipartite sites resulted in complete loss of exclusive nuclear expression.
The effects of mutations on the growth suppressive function of pRb2/p130 were then analyzed by recreating the NQ point mutation in PCR based site-directed mutagenesis in the full length pRb2/p130 protein in the pcDNA3 mammalian expression vector that drives expression by the constitutive cytomegalovirus (CMV) promoter. Each of the constructs was tagged at the carboxy-terminus with a single HA-epitope (hemagglutinin tag from Hemophilus influenzae) so that exogenous expression could be distinguished from that of the endogenous protein. Each of the mutants and the wild-type plasmids were expressed at approximately the same level as determined by immunoprecipitation and western blot analysis with the HA-epitope. Saos-2 cells were transfected with the mutant plasmids as well as vector alone and plasmids expressing the wild-type pRb2/p130 protein as controls, and their affects on cellular proliferation were measured by flow cytometry analysis. Mutations within either the upstream or downstream regions of the bipartite NLS had no significant effect on the growth suppressive activity of pRb2/p130. Their expression still led to G0/G1 phase growth arrest which was consistent with the localization data demonstrating that each of the bipartite signals can independently direct nuclear transport. The growth suppressive activity of the combined mutations in both bipartite sites was annihilated. Cells expressing this construct were clearly cycling. Addition of the c-myc major NLS to the carboxy terminus of these mutants by targeting their expression to the nucleus restored their G0/G1 growth arresting activity to that of the wild-type protein. This demonstrated that the reduced biological activity of pRb2/p130 conferred by these mutations was indeed by their disruption of the nuclear localization of pRb2/p130 and not by induction of gross conformational changes and/or the abrogation of binding to critical protein targets. Additionally, the growth suppressive effects of pRb2/p130 overexpression were not due to mere toxicity of the protein since point mutation in the protein effectively abrogated this activity. Furthermore, pRb2/p130 must be in the nucleus to regulate G1 progression. The same genetic analysis was also performed on primary Burkitt""s lymphoma and primary nasopharyngeal carcinomas, positive for Epstein-Barr virus (EBV). The two primary nasopharyngeal carcinomas displayed two heterologous point mutations in exon 20 which are also present in the human nasopharyngeal cell line, HONE-1. Moreover, insertions in exon 22 of the primary nasopharyngeal carcinoma and of HONE-1 were also found, which cause a frameshift with consequential loss of the nuclear localization signal. The point mutations and insertion present in two primary Burkitt""s lymphomas and in the two primary nasopharyngeal carcinomas are the same as those found in Daudi and HONE-1 cell lines, respectively.
Screening studies were also performed in several human lung carcinoma cell lines (H23, A549, H69, H82, and U1752) to determine if lack of Rb2/p130 protein observed in some tumor types correlated to a genetic mutation. Specifically, lung tumor cells were screened for mutations in exons 19 and 20 (B domain) as well as exons 21 and 22 (carboxy terminus) of the Rb2/p130 gene. Mutations were detected by SSCP analysis and confirmed by sequencing in 4 of 5 cell lines screened (H23, H69, H82, and U1752); A549 was found to be wild-type for the exons examined (no mutations). The same point mutations in exon 20 were found in 3 cell lines, corresponding to a coding region of the B domain. The substitution of an A for a G at nucleotide 3069 is a silent mutation; however, the same substitution at nucleotide 3074 changes Arg to Lys within a critical position in the bipartite nuclear localization signal of pRb2/p130. Both transitions were found in H69, H82 and U1752 small cell lung carcinoma lines. Thus, this region of the gene appears to be a mutational hot spot, the identification of which may be useful in developing a rapid diagnostic mutational screening assay.
In the H23 adenocarcinoma cell line, 5 point mutations, all G to A transitions, which altered 5 amino acids, were found in close proximity within exon 21 at nucleotides 3191, 3209, 3278, 3280, and 3285. Three polar basic Arg amino acids with their reactive guanidine groups were substituted with Lys (aa 1040) and Gln (aa 1047 and aa 1070). The alteration of a basic amino acid to a neutral amide-containing amino acid such as Gln, as well as the loss of the guanidine group, may have significant effects on the secondary and tertiary structure of the protein, as well as affecting protein interactions. Additionally, the substitution of a polar amino acid with a hydroxyl group such as Ser for the nonpolar aromatic Phe (aa 1074) may also significantly affect protein folding and functional interactions.
Primary patient samples of lung carcinoma were also screened for mutations in exon 19 through 22 of the Rb2/p130 gene. The Rb2/p130 gene was found to be mutated in exon 21 and/or 22 in 12 of 14 tumors analyzed. Insertion of an A in exon 22 causing a frameshift in either codon 11 and/or 1084 was found in 4 of 12 samples. This frameshift disrupts the bipartite nuclear localization of pRb2/p130 necessary for the exclusive nuclear expression of pRb2/p130 which is essential for the G0/G1 growth inhibitory function of pRb2/p130. A point mutation in codon 1083, which is within the upstream region of the bipartite nuclear localization and changes a critical Asn residue to either Phe or Thr was identified in 7 of the 12 mutated samples.
Thus, mutations disrupting the nuclear expression of pRb2/p130 appear to be a common mechanism of functional inactivation of this growth regulatory protein in primary tissues. Similar mutants in Rb/p105 have been shown to display reduced growth inhibition in vitro, probably due to insufficient levels of the protein in the nucleus (Zacksenhaus et al. Mol. Cell Biol. 1993 13:4588-4599). A similar phenomena occurs in the Rb2/p130 cytoplasmic mutants and is believed to predispose the cells to neoplastic transformation and/or give the cells a more aggressive or enhanced malignant phenotype. This is supported by immunohistochemical studies where the grade of the tumor inversely correlates with the expression level of pRb2/p130 (Baldi et al. Clinical Cancer Res. 1996 7:1239-1245). Accordingly, screening tumor cells to detect for mutations resulting in the loss of the NLS of Rb2/p130 serves as a means for determining the aggressive and malignant phenotype of the tumor. Further, screening of tumors to identify those with mutations in this gene is useful in identifying those tumors treatable by administration of Rb2/p130. By xe2x80x9ctreatablexe2x80x9d it is meant that administration of Rb2/p130 will inhibit the growth of this tumor.
Accordingly, the present invention provides methods for identifying aggressive and malignant tumors to be treated with a plasmid or vector comprising pRb2/p120 and methods for reversing the tumorigenicity by transfecting tumor cells with a plasmid or vector comprising pRb2/p130. In a preferred embodiment, Rb2/p130 cDNA is subcloned into an appropriate viral vector in accordance with methods of plasmid construction well known in the art and described for example by Claudio et al. (1994 Cancer Res. 54:5556-5560). As will be obvious to those of skill in the art upon this disclosure, however, appropriate vectors other than those described by Claudio et al. (1994) could be routinely selected. Selection of an appropriate vector is based upon adequate expression of the gene with minimal viral gene expression. One preferred embodiment of a vector useful in the present invention is the pAd.CMV-Link.1 adenoviral vector. Alternatively, using a transient three-plasmid expression system (plasmids encoding env, gag-pol, and Rb2/p130), the full length pRb2/p130 gene can be cloned into retroviral vectors. In another embodiment using plasmid pTet-tTak containing the gene for tetracycline transactivator under the control of the Tetp promoter, the full-length cDNA of human Rb2/p130 gene can be subcloned downstream of the Tetp promoter into the HindIII site of the UHD hyg BH plasmid. Yet another embodiment would involve using the MSCVPac plasmid where the full length cDNA sequence of Rb2/p130 was subcloned into the retroviral vector.
The vector encoding pRb2/p130 is then transfected into tumor cells. This can be accomplished using methods known to those of skill in the art. In a preferred embodiment, the viral vectors are administered to a mammal, preferably a human. In one embodiment, the vector containing the pRb2/p130 gene is administered to suppress growth of tumor cells and prevent tumor progression and metastasis through expression of the tumor suppressor gene. In this embodiment, the method of transfection will vary depending on the patient population under consideration. For example, in patients with operable tumors, the tumor area can be coated with the viral vector following resection by the surgeon to limit recurrence. Alternatively, the tumor mass can be injected with the vector prior to surgery. In patients with inoperable tumor masses, the patient can receive injections of the vector directly into the tumor mass if accessible, or it can be selectively injected intra-arterially to arteries directly feeding the tumor mass or inaccessible tumor (Claudio et al. 1990 Cancer 65:1465-1471; Claudio et al. 1992 Reg. Cancer. Treat. 4:180-187). Lung tumors can treated by inhalation of the vector.
It is preferred that the vectors be administered in a pharmaceutically acceptable carrier for injection such as a sterile aqueous solution or dispersion. Dose and duration of treatment is determined individually depending on the degree and rate of improvement. Such determinations are performed routinely by those of skill in the art.
The following nonlimiting examples are provided to further illustrate the present invention.